The Joint Tactical Information Distribution System (JTIDS) provides a secure, jam-resistance digital communication system for data and voice. JTIDS is a Time Division Multiple Access (TDMA) communication system operating at L-band frequencies. It supports the positive identification and precise location of all participating platforms, such as ships, aircraft, shore station facilities and the like. JTIDS is currently implemented by the United States Navy, the Joint Services, and forces of the North Atlantic Treaty Organization (NATO). The primary function of JTIDS within the Navy is to distribute tactical information in digital form to Navy tactical users such as ships and aircraft. JTIDS technology also locates and identifies subscribers with high precision.
JTIDS transmissions occure in time slots each consisting of a pulse train organized into a symbol signal structure, as shown in FIG. 1. Each symbol conveys five bits, in either one or two 6.4-.mu.sec pulses, in a 13-.mu.sec or 26-.mu.sec symbol, respectively. The five bits of each pulse are represented by a 32-chip code pattern obtained from cyclic shifts of a fixed 32-bit pattern. Frequency hopping and other spread spectrum techniques make JTIDS communications highly resistant to jamming, and data encryption makes it secure. As a digital system for both data and voice, JTIDS can handle large amounts of data. JTIDS transmits a frequency-hopping transmission pattern using 51 frequencies at 3-MHZ intervals, spread over the 969-1206 MHZ band, excluding two Identification Friend or Foe (IFF) subbands. JTIDS is also a multiple access system based on serially repeating, periodic epochs each comprised of a fixed number of assigned time slots during which messages may be received or transmitted. By the JTIDS standard, each time slot is 1/128 seconds (7.8125 msec) in duration. A frame is made up of 1,536 slots (12 seconds). An epoch is divided into 64 frames lasting 12.8 minutes. There are 128 separate nets, each having 98K time slots per epoch. A designated terminal, acting as a net time reference, causes the time slots in each net to coincide exactly. Thus, JTIDS may be more generally classified as a time division based system because messages can only be received or transmitted in predetermined time slots. For example, if a message reaches a JTIDS terminal outside a time slot designated as a "receiving time slot," the message will not be received.
A JTIDS system normally supports communications within an RF line-of-sight distance or approximately 500 nautical mile radius for an airborne based system. This distance limitation results from the fact that time division multiplexing requires that each time slot have sufficient null time to allow communications packets to be received before the beginning of the next time slot. Therefore, if the propagation path of the messages is great enough, the propagation time of the message causes the message to arrive at its destination Link-16 terminal across time slots or in the wrong time slot. In both cases, the message is not received.
In the past, training, integration testing, inter-operability testing, development testing, modeling, and simulation efforts for JTIDS communications system required that Link-16 terminals from many different platforms, as for example, ships and aircraft, be brought to one central location so they would be within the RF line-of-sight limit. However, such relocation was expensive and time consuming. Hence, very limited testing and training has been accomplished with the present JTIDS systems.
A need therefore exists for a way to communicatively couple time division based (TDB) networks which may be separated so far from each other that by the time a message transmitted by one network reaches a second network, the second network can no longer receive the message because the time slot in which the message can be received has timed out.